The present invention relates to a fuel reformer for obtaining hydrogen from a liquid fuel and a method for producing the fuel reformer, an electrode for use in an electrochemical device, and an electrochemical device, such as a fuel cell.
As a fuel for a fuel cell for use in automobile, pure hydrogen compressed and contained in a cylinder is mainly studied. On the other hand, as a fuel for a fuel cell for use in stationary apparatus, such as a cogeneration system, reformed gas obtained from methane gas or the like is studied. With respect to the fuel for fuel cell for use in small-size apparatus, methods using hydrogen stored in a hydrogen storage alloy or using a liquid fuel, such as methanol, have been proposed.
In a case where hydrogen stored in the hydrogen storage alloy is used in the fuel cell for a small-size apparatus, because of the weight of the hydrogen storage alloy, it is difficult for the whole of the system to achieve a weight energy density far larger than that of a conventional secondary battery. For this reason, in a case of expecting a fuel cell to exhibit a high weight energy density, the use of the currently used hydrogen storage alloy is unsuitable.
On the other hand, when a liquid fuel, such as methanol, is used in the fuel cell for a small-size apparatus, a direct methanol type in which methanol is directly introduced to a membrane electrode assembly (MEA) in the fuel cell, and a reforming method using a reformer provided independently of the MEA have been proposed.
In the direct methanol method, a mixture of methanol and water in a 1:1 ratio (molar ratio) theoretically serves as a fuel, but an ionic conductor is disadvantageously permeable to methanol and hence it is generally difficult to achieve such a methanol concentration, and, in practice, methanol is diluted to be in a concentration as small as 1 mol/l and then supplied to the MEA. However, in a case where methanol diluted to be in a concentration as small as 1 mol/l on the stage of a fuel tank is used in the fuel cell, it is suitable for the fuel cell to be expected to have a high weight energy density like a case of the fuel cell using pure hydrogen.
Therefore, a method in which methanol having a higher concentration is contained in the fuel tank and the methanol is diluted with water before being introduced to the MEA has been proposed. However, in this method, methanol and water must be fed individually with high accuracy, making a liquid feeding system complicated, and hence this method is not suitable for further miniaturization of the system.
On the other hand, in the method using a reformer independently provided, a methanol solution having a higher methanol concentration can be supplied as a fuel, and therefore the fuel energy density is not lowered and no complicated liquid feeding system is needed. However, in a case where a base material for the reformer is prepared by micro-fabrication of Si or the like as already proposed, the fabrication process is complicated, increasing the cost. Further, the reforming system provided independently of the MEA causes the volume to be larger.